In the third printing of the 1905 Chicago edition (in 1907), the original “I were about to write them” in this passage was changed to “I was about to write them”, thus eliminating the subjunctive use of the verb be in favor of the indicative. In a nearby chapter, in his editing for the 1837 edition, Joseph Smith made the same change for this phraseology:
In earlier English, the subjunctive was appropriate because of the implied negative in “I were about to ”. In both of these passages, Mormon did not end up writing down what he had first intended.
For other cases of “I was/were about to ”, the text has instances of only the indicative was; in two of the following cases (each marked with an asterisk), the speaker ends up not doing what he was going to do:
Thus the editing in 3 Nephi 26:11 and 3 Nephi 28:25 is consistent with the other examples in the text. In general, the original text allows for either the indicative or the subjunctive (see, for instance, the discussion under 2 Nephi 2:16). Thus the critical text will restore the subjunctive were in these two passages.
We also note here that for the 1907 impression of the 1905 LDS edition the were in the following relative clause (“which were engraven upon the plates of Nephi”) was changed to was. The 1911 LDS edition continued the was, but the 1920 LDS edition restored the original were.
Summary: Restore in 3 Nephi 26:11 and 3 Nephi 28:25 the two original instances of the subjunctive in “I were about to write something”; maintain in 3 Nephi 26:11 the plural were in the relative clause “which were engraven upon the plates of Nephi”.